System and method for direct-bonding of substrates

ABSTRACT

A method of forming a MEMS (Micro-Electro-Mechanical System), includes forming an ambient port through a MEMS cap which defines a cavity containing a plurality of MEMS actuators therein; and bonding a lid arrangement to the MEMS cap to hermetically seal the ambient port.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of bonding ofsubstrates. In particular, the invention relates to methods offabricating MEMS (Micro-Electro-Mechanical Systems) and other deviceswhich enable the operability and/or longevity of the devices.

MEMS and other devices often include two or more substrates either inclose proximity or bonded together. For example, in optical systems suchas digital projectors, a device may include an interference-baseddigital light display (DLD) package which includes two or moresubstrates to direct light to and from the DLD. Similar to a CRT, in arear-projection television, a DLD can be used in digital projectors forprocessing or generating an image from a source light.

One such DLD package is illustrated in FIG. 1. The package 100 includesa base substrate 120 with a driving electrode (not shown), a pixel plate110 which can move vertically, and a thin protective substrate ormembrane 130. In this arrangement a reflective coating is provided onthe pixel plate 110, and a partial reflective coating 131 is provided onthe bottom surface of the membrane 130. The protective membrane 130encloses a cavity in which the DLD pixel plate 110 is enclosed andallows light to pass therethrough.

To ensure reliability of the DLD package, the cavity must be essentiallyfree of contaminants and, in particular, essentially free of moisture.In this regard, such DLD packages are generally formed in a highlycontrolled environment so that moisture in the cavity is minimized.However, these methods can substantially increase the manufacturingcosts of the DLD package.

Thus, it is desirable to provide a reliable and inexpensive method andsystem of assembling such packages so that the effect of thecontaminants is minimized and the prolonged operation of the DLD packageis promoted.

SUMMARY OF THE INVENTION

One embodiment of the invention relates to a method of forming a MEMS(Micro-Electro-Mechanical System), comprising: forming an ambient portthrough a MEMS cap which defines a cavity, the cavity containing aplurality of MEMS actuators therein; and bonding a lid arrangement tothe MEMS cap to hermetically seal the ambient port.

Another embodiment of the invention relates to a MEMS package whereinthe MEMS package, comprises: a MEMS cap forming a cavity, the cavitycontaining a plurality of MEMS actuators therein; an ambient port formedthrough the MEMS cap; and a lid arrangement bonded to the MEMS cap, tohermetically close the ambient port.

In another embodiment, a digital projector includes a MEMS packagewherein the digital projector, comprises: a MEMS package whichcomprises: a MEMS cap forming a cavity, the cavity containing aplurality of MEMS actuators therein, the MEMS cap having an ambient porttherethrough; and a lid arrangement bonded to the MEMS cap.

In yet another embodiment, the invention relates to a MEMS package whichcomprises a MEMS cap means for forming a cavity which encloses aplurality of MEMS actuators therein, ambient port means formed in theMEMS cap for providing fluid communication with the cavity; a lidarrangement bonded to the MEMS cap; and means for protecting theplurality of MEMS actuators from contaminants trapped in the cavity.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and exemplary only, andare not restrictive of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art MEMS device; and

FIGS. 2 to 7 illustrate the formation of a MEMS device according to anembodiment of the invention.

FIGS. 8-11 illustrate the construction and arrangement of anotherembodiment of the invention.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

Referring to FIG. 2, a cross-sectional view of a package according to anembodiment of the invention is illustrated. In one embodiment, thepackage 200 includes an image processing device for use in a digitalprojector. The package 200 includes an exemplary digital light display(DLD) device with a plurality of MEMS actuators, such as pixel plates210, mounted on a support base 220. The pixel plates 210 may be arrangedin a two-dimensional array. Of course, other optical devices may beused, such as a liquid crystal display (LCD) or liquid crystal onsilicon (LCOS), for example. Such optical devices are well known tothose skilled in the art and do not require further discussion forpurposes of this application. While the package 200 in the illustratedembodiment is an optical device, it will be understood by those skilledin the art that the invention is not limited to optical devices and mayinclude other devices having two or more substrates and an enclosedcavity.

The support base 220 may be made of a variety of materials, such as asemiconductor or a non-conductive substrate, and may have a thicknessselected to provide sufficient strength to support the DLD pixel plates210. The material and thickness of the support base 220 is not limitingon the invention.

At an initial step illustrated in FIG. 3, the pixel plates 210 areencased by a protective membrane 230 mounted on the support base 220.The protective membrane 230 can be made of a variety of materials. Inone embodiment, the protective membrane 230 is silicon dioxide formedusing tetraethylorthosilicate (TEOS). The protective membrane 230 has apartially reflective coating 231 on its inboard or bottom surface. Thispartially reflective coating reflects a portion of the light and allowsthe remaining portion of incoming light to pass therethrough. The lightwhich passes through the coating 231 is reflected back from the pixelplates 210. The two reflections then cooperate to generate a desiredinterference effect which varies with the gap between the pixel plates210 and the protective membrane 230, and thus enables different colorsto be generated.

The protective membrane 230 may have a known refractive index (RI). Inthe case TEOS oxide is used in the fabrication of the silicon dioxide,the protective membrane 230 has an RI of approximately 1.5. In oneembodiment, the protective membrane 230 has a thickness of between 0.5and 2.0 microns at least in the region above the pixel plates 210.

The pixel plates 210 are positioned in an active area 202 of the package200. The active area 202 is configured to receive incoming light andprocess the light, via the pixel plates 210, to generate pixels whichcan be used to compose an image, for example. The package 200 extendsinto an inactive area 204 in which no light is actively processed by theMEMs but wherein it is overshadowed by a light absorbing layer such as alayer which is optically black or nearly 100% light absoring—see blacklayer 262 in FIG. 4 for example.

The protective membrane 230 forms a cavity 240 which may be filled witha sacrificial material to secure the pixel plates 210 in position and toprevent damage during transportation and/or assembly. In one embodiment,the sacrificial material in the cavity 240 is amorphous silicon. In theinactive area, a second cavity 250 is filled with a material. Thematerial in the second cavity 250 may also be amorphous silicon but isnot intended to be sacrificial. The second cavity 250 and the materialtherein can facilitate the setting of the height of the protectivemembrane 230 and thus the height of the boding surface acrosssustantially the entire chip.

Referring now to FIG. 3, clear-out holes are etched through theprotective membrane 230. The clear-out holes 232 may be etched using avariety of processes, such as chemical etching or laser etching using amask. In one embodiment, the holes are etched substantially above thestreets separating the rows and columns of the two-dimensional array ofpixel plates 210.

The clear-out holes 232 are used to remove the sacrificial material fromthe cavity 240. The removing of the sacrificial material may also beachieved in a variety of ways. These include chemical techniques such asusing liquid HF (hydrofluoric acid), TMAH (tetramethylammoniumhydroxide) or xenon difluoride (XeF₂). In the case wherein thesacrificial material is amorphous silicon or some form of silicon, theuse of xenon difluoride is possible. The process is not a plasma processbut a simple gas flowing technique wherein the gas is used to remove thesacrificial material from even very restricted small spaces and/orareas.

In addition to the clear-out holes 232, an ambient port 234 is etched inthe protective membrane 230 above an inactive area 204 of the package200.

Next, as illustrated in FIG. 4, a layer 260 of a material, such as anoxide, is formed on top of the protective membrane 230. The layer 260may be formed by any of a variety of ways. In one embodiment, the layer260 is deposited over the protective membrane 230, including theclear-out holes 232 and the ambient port 234.

In a particular embodiment, the above-mentioned black layer 262 oflight-absorbent material which is referred to a hide or HID, is formedon selected portions of the package. Specifically, the black layer 262is positioned above all regions except the pixel plates 210. Thelight-absorbent material of the black layer 262 serves to reduce oreliminate undesirable incidental light images reflected from the supportbase 220 or other components. A predetermined clearance may be providedaround the pixel plates 210 upon which the black layer is notpositioned. The clearance allows for an angle of incidence and ofreflection of the incoming light, for example.

A capping layer 264 is disposed over the black layer 262 in the mannershown in FIG. 4. In one embodiment, the capping layer 264 is formedusing TEOS oxide. Examples of other materials are PECVD (Plasma EnhancedChemical Vapor Deposition) silane based oxide or sputtered depositedsilicon oxide or silicon dioxide.

The capping layer 264 is then polished to Angstrom-level flatness viachemical-mechanical polishing (CMP), for example. The layers 260, 262,264 combine with the protective layer 230 to form a MEMS cap.

Referring now to FIG. 5, the portions of the layers 260, 262, 264 abovethe ambient port 234 are etched away to form an extension 234′ of theambient port 234. Again, the etching of these layers may be achieved viaa suitable etching technique.

It should be noted that the provision of the ambient port is such as toallow the pressure and atmosphere in the cavity 240 to be controlledright up until the ambient port and its extension are permanentlyclosed. This allows the pressure to be atmospheric, sub-atmospheric orabove atmospheric as desired. It also allows the content of theatmosphere to be controlled. For example, the cavity 240 can be filledwith nitrogen, or argon, for example, immediately prior the closure ofthe ambient port and thus improve MEMS functionality and/or reliablity.

Referring now to FIGS. 6 and 7, the bonding of a lid arrangement 299 tothe capping layer 264 of the MEMS cap is illustrated. As illustrated inFIG. 6, a black (hide) layer 272 can be additionally deposited onto anarea of a lid 274 corresponding substantially to the inactive area ofthe device 200, if deemed necessary.

The lid 274 may be formed of a glass. The black layer 272 can also bedeposited onto the surface of a cavity 275 formed in the lid 274, inwhich a contaminant absorbing material 276 such as water absorbingdesiccant or a getter which absorbs other forms of contaminants, ispositioned. The material 276 may, of course, be a getter or desiccant,or the like. In one embodiment, the material comprises silica gel(silicon dioxide) and is used to scavenge unwanted molecules, such aswater vapor molecules.

A layer of bonding substrate material 270, such as TEOS oxode, amorphoussilicon, phosphosilicate glass (PSG), glass frit, or silicon nitride isdeposited onto a bonding surface of the lid 274. The bonding substratematerial 270 may be deposited through a variety of methods such assputtering, chemical vapor deposition (CVD), or screen print, forexample. The layer of bonding substrate material 270 is relatively thinhaving a thickness on the order of between tens of an angstrom andtenths of a micron. In one embodiment, an anti-reflective coating isapplied to the opposite surface (the upper surface as seen in FIG. 6 2E)of the lid 274.

The bonding surfaces, including the capping layer 264 and the surface ofthe lid 274 with the bonding substrate material 270, may be polished forsmoothness. In this regard, the capping layer 264 and the surface of thelid 274 with the bonding substrate material 270 may be polished toAngstrom-level flatness via CMP, for example.

The bonding site (silanol group) density of at least one of the bondingsurfaces is increased to provide a more secure bonding of thesubstrates. The bonding site density may be increased through, forexample, plasma treatment and an optional wet treatment with eitherde-ionized water or SC1 (Standard Clean 1) chemistry. In this regard,the bond density of the capping layer 264 or the layer of bondingsubstrate material 270 on the lid 274 may be increased through any of avariety of methods including plasma treatment, ion implant and physicalsputtering. In a particular embodiment, the bonding site density isincreased for both surfaces. The increase in bonding site densityeffectively increases the bond strength of the pair.

In one embodiment, the bonding site density is increased by plasmatreating the bonding surfaces. This may be accomplished through, forexample, an ion beam sputtering process, a reactive ion etcher, strikingplasma onto the bonding surface, ion implantation or ion bombardment.The plasma treatment may use O₂, N₂ or Ar plasma or combinationsthereof, for example. The gases may be introduced serially or in variousmixures or combinations of mixtures. In one emboidment, O₂ is introducedintially for a short period (e.g. 15-25 seconds) to produce a littleoxygen ash and is immediately followed by N₂ or the like in the sametool or apparatus.

Following the plasma treatment, the bonding surfaces may be dipped inde-ionized water or SC1 chemistry for a period of time. In this regard,a minute or less is generally sufficient to increase the silanol group(Si—OH) density of the surfaces. For example, dipping for five minutesmay be sufficient. The surfaces may then be dried using, for example, aspin-rinse drier. Other methods of increasing bonding site density arewell known to those skilled in the art and are contemplated within thescope of the invention.

Referring now to FIG. 7, the bonding surfaces are fusion bonded at roomtemperature. The fusion bonding may be accomplished by holding thebonding surfaces together while applying a compression force. Theincreased bonding site density allows the fusion bonding to be performedat substantially room temperature, rather than typical fusion bondingprocesses which may require annealing temperatures as high as 900° C.“Room temperature,” as used herein, includes temperatures rangingbetween approximately 15 and approximately 40° C.

In one embodiment, the package 200 is annealed. In one embodiment, thelid 274 formed of glass with a thin layer 270 of TEOS oxide bonded tothe capping layer 264 formed of TEOS oxide, and the package 200 isannealed at approximately 100-200° C. for approximately two hours, todevelop a final bond strength and hermeticity level required.

Thus, the capping layer 264 and the lid 274 are bonded to each otherwith no need for an anti-reflective coating on the bonding surfaces. Theincreasing of the silanol-group density through plasma treatment andpost-bond annealing provide a bond of sufficient strength to secure thecapping layer 264 and the lid 274 to each other. In one embodiment, thelid 274 is formed of glass and has a thickness of between 0.5 and 3 mmfor example.

Further, the package 200 may be made hermetically sealed by assuringthat the lid 274 is sufficiently thick to prevent moisture or gasmolecules to penetrate therethrough. For example, this hermeticalsealing can be arranged to meet the military standard MIL STD 883C.

The lid arrangement 299 is positioned such that the cavity 275containing the absorbent contaminant removing material 276 is in fluidcommunication with the cavity 240 containing the pixel plates 210 viathe ambient port 234. Thus, any vapor molecules in the environment ofthe pixel plates 210 are scavenged and absorbed by the material 276.

FIGS. 8-11 shows a further embodiment of the invention wherein theclear-out holes 232 are omitted and the fabrication of the DLD device isadvance to the degree where the layers 260, 262, 264 are disposed on theprotective membrane 230 with the protective sacrificial material stillin place in the cavity 240 (see FIG. 8). The ambient port 234 is thenformed (see FIG. 9).

At this stage it is possible to flow a gas (see flow arrows in FIG. 10),such as xenon difluoride (XeF₂) or the like, into the cavity 240 andremove the sacrificial material. This has the advantage of leaving thesacrificial material in position until the last minute and protectingthe pixel elements from the high temperature (e.g. 400° C.) and/orplasma conditions which are involved with the formation of one or moreof the layers 260, 262 and 264 and the formation of the ambient port.The ambient port 234 is then closed via the application of the cap inthe manner shown in FIG. 11.

In a further embodiment, it is possible to introduce an anti-stictionmaterial into the cavity 240. One example of such an anti-stictionmaterial is FOTS (florininated octytrichlorosilane). This anti-stictionmaterial can be used in combination with a contaminant absorbingmaterial which is disposed in a recess portion of the lid 274, or can beused alone. That is to say, by using an anti-stiction material withhydrophobic properties, the pixel elements can be protected from thedeleterious effects of moisture and the like in addition to beingprevented from sticking due to the lubricating nature of the material.

The contamination of the upper surface of layer 264 by the anti-stictionmaterial which occurs when introducing the material into the cavity 240,is conveniently removed by briefly introducing oxygen into the plasmachamber to produce a quick O₂ ash and then switching to nitrogen at thetime of performing the surface plasma which is used to active thesurface in preparation for bonding the lid into place. The removal ofthe anti-stiction material form the external surfaces does not markedlyeffect the interior of the cavity and no deleterious effect is had tothe anti-stiction material within the cavity 240 per se.

At this stage it is possible to adjust the pressure and content of theatmosphere within the cavity, if so desired, and then bond the cap intoposition to hermetically close the ambient port.

It will be appreciated, the use of the anti-stiction material can beused alone as can the use of the contaminant absorbing material.Nevertheless, these two protective/prophylactic techniques can also beused in combination if so desired

The foregoing description of embodiments of the invention have beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and modifications and variation are possible in light of theabove teachings or may be acquired from practice of the invention. Thedisclosed embodiments were chosen and described in order to explain theprinciples of the invention and its practical application to enable oneskilled in the art to utilize the invention in various embodiments andwith various modification as are suited to the particular usecontemplated. It is intended that the scope of the invention be definedby the claims appended hereto and their equivalents.

1. A MEMS package, comprising: a support base; a MEMS cap forming acavity with the support base, the cavity containing a plurality of MEMSactuators disposed on the support base therein; an ambient port formedthrough the MEMS cap; and a lid arrangement bonded to the MEMS cap, tohermetically close the ambient port; a recess formed in the lid, therecess being configured to be in fluid communication with the ambientport; and a contaminant absorbing material disposed in the recess.
 2. AMEMS package as set forth in claim 1, wherein the ambient port is formedto open into the cavity at a location which is adjacent to and offsetwith respect to a location wherein the plurality of MEMS actuators aredisposed.
 3. A MEMS package as set forth in claim 1, further comprisinga quantity of anti-stiction material disposed in the recess and enclosedtherein by the lid.
 4. A MEMS package as set forth in claim 1, furthercomprising an atmosphere enclosed in the cavity, the atmosphere having acontent and pressure determined immediately prior closure of the ambientport by the lid.
 5. A MEMS package set forth in claim 1, wherein theMEMS actuators include pixel plates forming a digital light display. 6.A MEMS package set forth in claim 1, wherein the contaminant absorbingmaterial is a getter or a desiccant.
 7. A MEMS package set forth inclaim 1, wherein the contaminant absorbing material includes silicondioxide.
 8. A MEMS package, comprising: a support base: a MEMS cap meansfor forming a cavity with the support base which encloses a plurality ofMEMS actuators disposed on the support base therein, ambient port meansformed in the MEMS cap for providing fluid communication with thecavity; a lid arrangement bonded to the MEMS cap; and means forprotecting the plurality of MEMS actuators from contaminants trapped inthe cavity; wherein the protecting means comprises an absorbent materialreceiving means formed in the lid arrangement for receiving acontaminant absorbing material therein, the absorbent material receivingmeans being positioned to allow fluid communication between thecontaminant absorbing material and the plurality of MEMS actuatorsthrough the ambient port.
 9. The MEMS package as set forth in claim 8,wherein the contaminant absorbing material comprises a getter or adesiccant.
 10. The MEMS package as set forth in claim 8, wherein thecontaminant absorbing material comprises silicon dioxide.
 11. The MEMSpackage as set forth in claim 8, wherein the protective means comprisesa quantity of anti-stiction material which is introduced into the cavitythrough the ambient port.